When people don't get enough sleep they tend to eat more, causing them to gain weight. This was the conclusion of a new study led by University of Colorado Boulder in the US that was published online this week in the Proceedings of the National Academy of Sciences.

The researchers found that volunteers who slept only 5 hours a night during a working week put on nearly two pounds (0.8 kg) in weight if they had unlimited access to food.

There is already a lot of evidence linking insufficient sleep with obesity, write the researchers in their background information, but we know little about how repeated nights of insufficient sleep affect energy balance.

Kenneth Wright, director of the Sleep and Chronobiology Laboratory at the University of Colorado Boulder (CU-Boulder), says in a statement:

"Just getting less sleep, by itself, is not going to lead to weight gain. But when people get insufficient sleep, it leads them to eat more than they actually need."

Weight Loss Programs Should Include Healthy Sleep

In their study, Wright and colleagues found that while staying awake longer requires more energy, the extra calories consumed are more than the extra calories burned.

They suggest encouraging people to get the sleep they need could help fight the obesity epidemic, but emphasize the problem is not that simple:

"I don't think extra sleep by itself is going to lead to weight loss," says Wright, "But I think it could help."

"If we can incorporate healthy sleep into weight-loss and weight-maintenance programs, our findings suggest that it may assist people to obtain a healthier weight," he proposes, although he adds that more research is needed to test that idea.

The Study

For their study, Wright and colleagues invited 16 young, lean, healthy adult volunteers to live in a "sleep suite" at the University of Colorado Hospital for about two weeks.

The "sleep suite" offers a quiet and peaceful setting for sleeping, but it is also a controlled environment. The researchers decide when lights go on and off, and the sealed room allows them to measure the oxygen that participants breathe in and the carbon dioxide they breathe out from which they can work out how much energy they are using.

The first three days were spent establishing the baseline measures. All volunteers had the chance to sleep 9 hours a night and they were given only enough food to maintain weight.

After that they were put into two groups for five days: one group had their sleep opportunity restricted to 5 hours and the other continued with 9 hours. Also, their food provision changed so they could eat larger meals and help themselves outside mealtimes to snacks ranging from yogurt and fruit to potato chips and ice cream.

Then, the researchers switched the groups over for the next five days.

The Findings: Restricted Sleep Followed By Overeating, Weight Gain

The results showed that when their sleep was restricted to 5 hours a night, the volunteers on average burned about 5% more calories than when they could sleep up to 9 hours a night, but they consumed 6% more calories.

Also, while on restricted sleep, they ate smaller breakfasts but ate more snacks after the evening meal, to the extent that the total calories consumed in evening snacks was more than the calories in any one meal.

Moving to a restricted sleep schedule resulted in weight gain of nearly 2 pounds (0.8 kg), and also caused a shift in the biological clock, leading to an "earlier circadian phase of wake time", write the authors.

But moving the other way, from restricted sleep to sufficient sleep, reduced energy intake, especially of fats and carbohydrates, and led to a slight drop in weight.

There were also differences between the male and female volunteers. When they could sleep up to 9 hours a night men tended to put on weight whereas women maintained their weight, "where as insufficient sleep reduced dietary restraint and led to weight gain in women".

The researchers suggest their findings add to the growing evidence that eating too much at night may lead to weight gain.

Wright says their study shows when people don't get enough sleep they eat at night, when their bodies are not equipped for taking in food.

He and his colleagues are now planning another study that looks at the effect of when people eat, as opposed to what they eat, on weight.

An animal study that has shed some light in this area was published in Nature Medicine in 2012, where scientists described how deleting a clock gene leads to obesity in mice, and also causes a change in the timing of their normal eating schedule.

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